This invention relates to a process for making insecticidal compounds. More particularly, it refers to an improved process for making N-methylcarbamates.
The following patents describe methods of making N-methylcarbamate insecticides or the methyl isocyanate intermediate for making N-methylcarbamates.
1. U.S. Pat. No. 3,576,834 discloses a method for the preparation of methomyl insecticide as shown below. ##STR1##
2. U.S. Pat. No. 3,658,870 teaches a similar method for the preparation of oxamyl insecticide.
3. U.S. Pat. No. 3,217,037 describes insecticides such as aldicarb.
4. U.S. Pat. Nos. 3,474,170 and 3,474,171 describe carbofuran as an insecticide.
5. U.S. Pat. No. 2,903,478 describes novel insecticides such as carbaryl.
6. U.S. Pat. No. 4,255,350 describes a method for preparing methyl isocyanate wherein a mixture of potassium cyanate, potassium iodide and DMF in m-xylene is treated with methyl chloride at 120.degree. C.
7. U.S. Pat. No. 4,206,136 teaches the preparation of methyl isocyanate by treatment of potassium cyanate with dimethyl sulfate in the presence of calcium oxide and/or calcium chloride in a refluxing solvent such as dichlorobenzene.
Several methods, in addition to those set forth in the aforementioned references, are known in the art for the preparation of N-methylcarbamates. Perhaps the most widely-known procedure involves treatment of the appropriate starting material, ROH, as hereafter identified with methyl isocyanate. This process suffers from the disadvantage of requiring the handling and storage of methyl isocyanate, which is both burdensome and extremely hazardous, as well as being comparatively expensive on an industrial scale.
Methyl isocyanate can be synthesized in several ways including the two procedures taught in references 6 and 7 above or thirdly, the reaction of phosgene with monomethylamine.
The first and second processes suffer several drawbacks including the need for high reaction temperatures, expensive and/or toxic cosolvents (DMF is a known embryotoxin), gaseous reactants (methyl chloride), and additives such as potassium iodide, calcium oxide or calcium chloride.
The third process suffers from several disadvantages. First, both phosgene and monomethylamine are gases at ambient temperature, and phosgene is extremely toxic. Second, this reaction evolves two moles of hydrogen chloride for each mole of reactant consumed; disposal of this strongly acidic and corrosive by-product can be costly.
The most serious prior art problem occurs when large quantities of methyl isocyanate must be isolated and transported for long distances, before conversion into the N-methylcarbamate insecticide. The extreme toxicity of methyl isocyanate is now well known throughout the world. A process needed to be developed which produced N-methylcarbamates at low cost without the necessity of isolating methyl isocyanate during the processing steps.